Structural characterisation of a MAPR-related archaeal cytochrome b_5M protein

We recently reported that the membrane-associated progesterone receptor (MAPR) protein family (mammalian members: PGRMC1, PGRMC2, NEUFC and NENF) originated from a new class of prokaryotic cytochrome b₅ (cytb₅) domain proteins, called cytb_5M (MAPR-like). Relative to classical cytb₅ proteins, MAPR and ctyb_5M proteins shared unique sequence elements and a distinct heme-binding orientation at an approximately 90° rotation relative to classical cytb₅, as demonstrated in the archetypal crystal structure of a cytb_5M protein (PDB accession number 6NZX). Here, we present the crystal structure of an archaeal cytb_5M domain (Methanococcoides burtonii WP_011499504.1, PDB:6VZ6). It exhibits similar heme binding to the 6NZX cytb_5M, supporting the deduction that MAPR-like heme orientation was inherited from the prokaryotic ancestor of the original eukaryotic MAPR gene.